1. Field of the Invention
This invention relates to a method for coating inner surface of a metal tube with a powdery paint and an apparatus therefor. More particularly, this invention relates to a method for coating inner surface of a metal tube with a heat-fusible resin powdery paint and an apparatus therefor.
2. Prior Art
A method and an apparatus for coating inner surface of a metal tube with a powdery paint were disclosed in Japanese Patent Kokai No. 8-89875. According to this invention, a heated metal tube is placed in a fluidized bed dipping tank in which powdery paint floats while retaining it around it's horizontal axis; and air stream is introduced into the metal tube, whereby the outer surface and the inner surface of the metal tube are simultaneously coated.
According to this technique, both the inner and outer surfaces of a metal tube are coated simultaneously with the same paint. There are some cases where an outer surface need not be coated or must be coated with other paint particularly when the tube is made of steel. In these cases, the above-mentioned method is not applicable.
There is another method for coating an inner surface of a steel tube, in which a powder spreader is inserted into the tube while gradually heating the tube. However, this method is not only time-consuming and unacceptable to mass production, but also is not applicable to tubes with a small diameter (e.g. 100 mm or less) because the powder spreader cannot enter into the tube. In addition, when the tube is long, a substantial time is required for coating and thus the tube is cooled during coating operation.
There is a demand for coating only inner surfaces of various pipes used in ships for passing oils and seawater through the pipes. Such inner surface coating has been conventionally effected by a liquid paint spraying. In this case, however, the thickness of a coat obtainable by one coating operation is as thin as about 10 .mu., and a coat of at most about 30 .mu. thick is obtainable by repeating coating operation three times. By such coating, pinholes are inevitably produced in the inner surface of a tube. Moreover, the resulting coat is low in adhesion and thus easily peeled off, which makes long-term use of the coated tube impracticable.